This invention relates to determining the location of a wireless mobile unit. This invention is especially, but not exclusively, suited for location determination of a cellular mobile unit where information from which its location is determined is available from only a single antenna site.
The location of cellular communication devices for E911 calls must be made available by service providers in the United States and must have a location accuracy as specified by the Federal Communications Commission. For Universal Mobile Telecommunication Systems (UMTS) there are primarily four methods for locating a mobile communication device as defined in the 3GPP standards. One of these methods requires the mobile communication device to have global positioning satellite (GPS) capabilities. It will be appreciated that not all mobile communication devices have such capabilities. Thus, this method is not generally applicable to all mobile units.
Another method for location of a mobile unit is referred to as cell-ID. In its simplest form, the cell with the strongest signal strength for the subject mobile unit is identified and the position of this cell is returned as the mobile unit's position. Because the mobile unit could be anywhere within its coverage region, the method in this basic form is not very accurate. Variations of this method have been implemented to increase the accuracy of the location of the mobile unit as described in FIG. 1.
FIG. 1 illustrates a cellular communication system 10 that includes representative cellular antenna sites 12 and 14. A cellular handset 16 is located within the coverage area of both sites 12 and 14. A radio network controller (RNC) 18 is coupled to antenna sites 12 and 14 by communication lines 20 and 22, respectively. A mobile switching center (MSC) 24 is coupled to the RNC 18 and to a further communication network 26. A location measurement unit (LMU) 28 is coupled to the RNC 18 and is utilized to process information supplied by the infrastructure equipment and/or subject mobile unit in order to make a determination of the location of the mobile unit. Once this determination is made by the LMU 28, the determined location can be, for example, transmitted to an emergency services center that services a corresponding E911 call from the mobile unit.
Antenna site 12 is divided into three sectors 30, 31 and 32 each served by a directional antenna. Similarly, antenna site 14 is divided into sectors 34, 35 and 36. To determine the location of cellular handset 16, the cellular handset is requested to provide two types of measurement data by a standard protocol (RRC). The cellular handset provides a receive-transmit time difference (RxTx) value and a path loss measurement representing the loss in power of signals from the antenna site of the cell to the cellular handset. In this example, these measurements are provided by the cellular handset 16 for both cell sites 12 and 14. The respective RxTx values in combination with the corresponding round trip time (RTT) values from the cells are used to calculate corresponding circles 38 and 40 around the respective center of the cells. The intersection of the circles is used to determine the position of the cellular handset. The information utilized to make the location determination is transmitted to the LMU 28 which calculates the circles 38 and 40, and then determines the point of intersection of the circles which is determined to be the location of the cellular handset. While this technique provides adequate location accuracy, it cannot be used in situations where such location information is available for a mobile unit from only a single cellular site since only a single circle would be defined. Thus, there exists a need for an improved method that can determine the location of a mobile unit with sufficient accuracy, even when location information is available only for a single cellular site.